✨ implement unified tile system (Issue #14)

- Tree seedlings: plant tree_seed on grass via farming tool; two-stage
  growth (sprout → sapling → young tree, ~1 min/stage); matures into
  a harvestable FOREST resource tile
- Tile recovery: Nisse chops start a 5-min DARK_GRASS→GRASS timer;
  terrain canvas updated live via WorldSystem.refreshTerrainTile()
- New TreeSeedlingSystem manages sprites, growth ticking, maturation
- BootScene generates seedling_0/1/2 textures procedurally
- FarmingSystem adds tree_seed to tool cycle (F key)
- Stockpile panel shows tree_seed (default: 5); panel height adjusted
- StateManager v5: treeSeedlings + tileRecovery in WorldState
- WorldSystem uses CanvasTexture for live single-pixel updates

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/config.ts

@@ -46,5 +46,11 @@ export const VILLAGER_NAMES = [
   'Orla','Pike','Quinn','Rook','Sera','Tull','Uma','Vex',
 ]
 
-export const SAVE_KEY = 'tg_save_v4'
+export const SAVE_KEY = 'tg_save_v5'
 export const AUTOSAVE_INTERVAL = 30_000
+
+/** Milliseconds for one tree-seedling stage to advance (two stages = full tree). */
+export const TREE_SEEDLING_STAGE_MS = 60_000   // 1 min per stage → 2 min total
+
+/** Milliseconds before a bare DARK_GRASS tile (after tree felling) reverts to GRASS. */
+export const TILE_RECOVERY_MS = 300_000        // 5 minutes